A superelastic alloy has an extremely wide elasticity range when compared to other metal materials at a temperature not lower than a reverse transformation temperature, and has a property of recovering an original shape even when being deformed. The superelastic alloy is expected to be applied to a medical field and medical instruments such as dental braces, a clasp, a catheter, a stent, a bone plate, a coil, a guide wire, and a clip by use of these characteristics.
The superelastic alloy was investigated with respect to various alloy types based on information about a shape-memory alloy. Examples of a superelastic alloy currently best known in terms of practicability include a Ni—Ti-based shape-memory alloy. The Ni—Ti-based shape-memory alloy has a reverse transformation temperature of 100° C. or less, and may exhibit superelasticity at a human body temperature, and thus is considered to be applicable to a medical instrument in terms of characteristic. However, the Ni—Ti-based shape-memory alloy contains Ni which involves concern about biocompatibility due to metal allergy. Biocompatibility is considered to be a fatal problem when application to a medical field is taken into consideration.
In this regard, an alloy material which may exhibit superelastic property while being Ni-free is developed. For example, Patent Document 1 discloses a Ti alloy formed by addition of Mo and one of Al, Ga, and Ge to Ti. In the Ti alloy, Mo is added as an additional element having β-phase stabilizing action of Ti, and Al, Ga, or Ge having excellent biocompatibility are added among additional elements having α-phase stabilizing action. Superelastic property is exhibited by appropriate adjustment of concentrations of the additional elements. Additionally, it is reported that various Ti-based alloys such as a Ti—Nb—Al alloy, and a Ti—Nb—Sn alloy may exhibit superelastic property.